In the code review Victor asked the following question:

> I'm a little bit surprised that you need to pass so much options which are specific to the platform. GCC doesn't have a generic option "hello, please compile for my architecture?"

ARM defines two incompatible floating point ABIS that specify the functions calling conventions, whether to use integer registers (soft floating point ABI) or the floating point registers (hard floating point ABI).  The compiler must be told which one to use and whether the hardware FPU may be used if present when the soft floating point ABI has been chosen. This last case is set with the command line option '-mfloat-abi=softfp'. The following two documents describe this feature in details:
  https://wiki.debian.org/ArmHardFloatPort/VfpComparison
  https://wiki.debian.org/ArmHardFloatPort

The following command prints the command line options, including implicitly predefined ones of the clang compiler in android-ndk-r13b and shows that the default is '-mfloat-abi=soft' for armv7:

$ ./clang -x c < /dev/null -dM -E - -target armv7-none-linux-androideabi -###
Android clang version 3.8.256229  (based on LLVM 3.8.256229)
Target: armv7-none-linux-android
Thread model: posix
InstalledDir: /opt/android-ndk/toolchains/llvm/prebuilt/linux-x86_64/bin/.
 "/home/opt.symlink/android-ndk-r13b/toolchains/llvm/prebuilt/linux-x86_64/bin/clang" "-cc1" "-triple" "armv7-none-linux-android" "-E" "-disable-free" "-disable-llvm-verifier" "-main-file-name" "-" "-mrelocation-model" "pic" "-pic-level" "1" "-mthread-model" "posix" "-mdisable-fp-elim" "-fmath-errno" "-masm-verbose" "-mconstructor-aliases" "-fuse-init-array" "-target-cpu" "cortex-a8" "-target-feature" "+soft-float-abi" "-target-abi" "aapcs-linux" "-mfloat-abi" "soft" "-target-linker-version" "2.24" "-dwarf-column-info" "-debugger-tuning=gdb" "-resource-dir" "/home/opt.symlink/android-ndk-r13b/toolchains/llvm/prebuilt/linux-x86_64/bin/../lib64/clang/3.8.256229" "-internal-isystem" "/usr/local/include" "-internal-isystem" "/home/opt.symlink/android-ndk-r13b/toolchains/llvm/prebuilt/linux-x86_64/bin/../lib64/clang/3.8.256229/include" "-internal-externc-isystem" "/include" "-internal-externc-isystem" "/usr/include" "-fdebug-compilation-dir" "/opt/android-ndk/toolchains/llvm/prebuilt/linux-x86_64/bin" "-ferror-limit" "19" "-fmessage-length" "100" "-femulated-tls" "-fallow-half-arguments-and-returns" "-fno-signed-char" "-fobjc-runtime=gcc" "-fdiagnostics-show-option" "-fcolor-diagnostics" "-dM" "-o" "-" "-x" "c" "-"

Note that the system libffi must also be compiled with these same flags. The bundled libffi is deprecated in 3.6 by issue 27976 and has been removed in 3.7 by issue 27979.